Many resource-poor countries must rely on simplified antiretroviral (ARV) regimens to prevent HIV mother-to- child transmission (MTCT). Unfortunately, these regimens are not fully effective and offer little protection against HIV transmission by breastfeeding, which accounts for at least 1/3 of all pediatric HIV infections. This situation is likely to improve, since two recent studies show that the risk of post-natal HIV transmission can be greatly reduced by providing breastfeeding infants with extended daily prophylaxis with nevirapine (NVP) or NVP plus zidovudine (ZDV). Therefore, extended NVP is likely to become a key component in regimens used to prevent MTCT in resource-poor countries where breastfeeding is critical for infant health. Unfortunately, data from one study shows that when infants are HIV-infected despite prophylaxis, those who received single dose NVP (sdNVP) plus up to 6 weeks of daily NVP had higher rates of NVP resistance than infants who received sdNVP alone;NVP resistance was also more likely to persist over time in infants who received the extended NVP regimen. In resource-poor settings, most first-line ARV treatment regimens for HIV-infected children include NVP. Therefore, the presence of NVP-resistant HIV strains in infants who received extended NVP prophylaxis may significantly reduce their chance of future ARV treatment success. The hypothesis of the proposed studies is that the addition of extended ZDV to extended NVP prophylaxis will reduce emergence and persistence of NVP resistance in breastfeeding infants who become HIV-infected despite having received prophylaxis. We will test this hypothesis by analyzing samples and data from the Post-Exposure Prophylaxis of Infants in Malawi trial (PEPI-Malawi). In PEPI-Malawi, most women received sdNVP in labor, and 3,352 infants were randomized to one of three study arms: (1) the control arm [sdNVP plus 1 week of daily ZDV], (2) the extended NVP arm [control regimen plus daily NVP to 14 weeks of age], or (3) the extended NVP/ZDV arm [control regimen plus daily NVP and daily ZDV to 14 weeks of age]. At 9 months, the estimated risk of post- natal HIV transmission among infants who were HIV-uninfected at birth was 10.6% in the control arm, 5.2% in the extended NVP arm (p<0.001), and 6.4% in the extended NVP/ZDV arm (p=0.002). The reduction in HIV transmission in the extended NVP and extended NVP/ZDV arms was still observed at 24 months. In the proposed studies, we will analyze emergence and persistence of NVP resistance in infants in PEPI-Malawi who were HIV-uninfected at birth, but became HIV-infected by 14 weeks of age. We will compare NVP resistance in the extended NVP arm and the extended NVP/ZDV arm, and will identify factors that influence emergence and persistence of NVP resistance in infants. We will also assess whether NVP resistance influences infant health. This information will facilitate the design and implementation of programs for prevention of post-natal MTCT in Africa and elsewhere. PUBLIC HEALTH RELEVANCE: These studies will evaluate antiretroviral drug regimens that reduce the risk of HIV transmission during breastfeeding. These studies will facilitate the design and implementation of programs for prevention of HIV mother-to-child transmission in resource-poor settings.